const lastDate = new Date('2020/12/31 23:59"59');
const mainTag = document.querySelector('main');
setInterval(()=>{
    const nowTime = new Date();
    const interval = lastDate - nowTime;
    const datCount = Math.floor(interval/1000/60/60/24);
    const datHours = Math.floor(interval/1000/60/60);
    const datMinutes = Math.floor(interval/1000/60)
    const datMinutes =Math.floor(interval/1000);

    mainTag.textContent =`距离今年最后一天还有:
    ${datCount}天 ${datHours}时 ${datMinutes} 分 ${datSeconds}秒`;
},1000);
